ar71xx: fix kernel relocate stub parallel build issue

Use per-kernel directory

Signed-off-by: Felix Fietkau <nbd@nbd.name>

target/linux/ar71xx/image/Makefile

@@ -79,15 +79,16 @@ define Build/loader-okli
 endef
 
 define Build/relocate-kernel
-	rm -rf $(KDIR)/relocate
-	$(CP) ../../generic/image/relocate $(KDIR)
-	$(MAKE) -C $(KDIR)/relocate KERNEL_ADDR=$(KERNEL_LOADADDR) CROSS_COMPILE=$(TARGET_CROSS)
+	rm -rf $@.relocate
+	$(CP) ../../generic/image/relocate $@.relocate
+	$(MAKE) -j1 -C $@.relocate KERNEL_ADDR=$(KERNEL_LOADADDR) CROSS_COMPILE=$(TARGET_CROSS)
 	( \
-		dd if=$(KDIR)/relocate/loader.bin bs=32 conv=sync && \
+		dd if=$@.relocate/loader.bin bs=32 conv=sync && \
 		perl -e '@s = stat("$@"); print pack("N", @s[7])' && \
 		cat "$@" \
 	) > "$@.new"
 	mv "$@.new" "$@"
+	rm -rf $@.relocate
 endef
 
 define Build/copy-file